1. Field of the Invention
The present invention relates to a process for manufacturing a membrane module containing at least two groups of hollow-fiber membranes arranged in layers and substantially parallel to each other, and a module of this type.
2. Discussion of the Related Art
In various processes in the pharmaceutical or chemical industries, it is necessary to conduct multi-stage extraction processes in which a component dissolved in an aqueous phase, for example, is first extracted using an organic phase and this component is subsequently separated from the organic phase using a second aqueous phase. Such multi-stage processes can be combined as single-stage processes in membrane modules containing two groups of hollow-fiber membranes, wherein the feed stream flows through the hollow-fiber membranes of one group and the strip stream ultimately containing the separated component flows through the hollow-fiber membranes of the second group. The space surrounding the hollow-fiber membranes is filled with the extraction fluid that transports the component to be extracted from the hollow-fiber membranes of the first group to the hollow-fiber membranes of the second group. The liquid enclosing the hollow-fiber membranes acts as a liquid membrane. Such processes are also described in the literature as CLM (contained liquid membrane) separation processes (see for example Majumdar et al., AlChE Journal, Vol. 34 (1988), No. 7, pp. 1135-1145; Sengupta et al., AIChE Journal, Vol. 34 (1988), No. 10, pp. 1698-1708; Basu et al., J. of Membrane Science, Vol. 75 (1992), pp. 131-149). Using these separation processes, components can be separated from both liquid or gaseous media. The membrane modules generally described in the cited literature have two groups of hollow-fiber membranes that in the middle section of the housing are arranged adjacently and parallel to each other. At the ends of the housing, the ends of the hollow-fiber membranes are separated by group, for example via Y-shaped end pieces with corresponding inlet or outlet arrangements.
Similar membrane modules having two groups of hollow-fiber membranes are also known from the field of biotechnology. In U.S. Pat. No. 5,043,260, a membrane module is described for breeding and preserving hepatocytes for an artificial liver, the module having in its middle section, which has a round cross-section, a bundle of two intermingled groups of hollow-fiber membranes arranged parallel to one another. The ends of the hollow-fiber membranes are separated by group and embedded into Y-shaped end pieces at the ends of the module housing. The hepatocytes are immobilized on the external surfaces of the hollow-fiber membranes; nutrient liquid is fed in via the hollow-fiber membranes of the first group and waste products are removed via the hollow-fiber membranes of the second group.
EP-A-514-021 discloses membrane modules that are suitable for CLM separation processes, as well as a process for manufacturing the modules. Initially, for example, a single hollow-fiber membrane is laid on wave-shaped and intersecting paths in multiple layers on a suitable frame, and in this manner a bundle is produced. In a further step, the loops producing during the laying process are cut through at the ends of the bundle and the end pieces produced following cutting are separately combined. In this manner, two groups of intermingled hollow-fiber membranes are produced in the bundle; when the bundle is removed from the frame, the hollow-fiber membranes in the bundle have a sinusoidal, in part intersecting arrangement.
The bundles obtained hereby are drawn into tube-shaped housing middle sections and the end pieces are directed separately into the arms of Y-shaped end pieces, into which they are embedded. The arms of the Y-shaped end pieces are joined to appropriate inlet and outlet arrangements such that two different liquids can flow separately through the two groups of hollow-fiber membranes.
These membrane modules according to EP-A-514 021 exhibit the disadvantage that the hollow-fiber membranes of the two groups have varying spacing from each other over the entire extent of the housing, and zones may result with different filling ratios of the hollow-fiber membranes. Furthermore, drawing of the bundles into the housing components proves problematic, especially when the housings are relatively long and especially when higher overall filling ratios of the hollow-fiber membranes in the housing are to be attained. Finally, while a membrane module according to EP-A-514 021 does contain two groups of hollow-fiber membranes capable of being fed independently by fluids, the hollow-fiber membranes of the two groups must always be the same due to the manufacturing process of the module.
A concept similar to that of EP-A-514 021 is pursued in EP-A-515 034. The modules according to EP-A-515 034 also contain hollow-fiber membrane arrangements with which two groups of hollow-fiber membranes are associated, wherein their ends, separated by group, are directed through the arms of Y-shaped end pieces. These arrangements are constructed of superimposed layers made from pieces of hollow-fiber membrane fabrics of appropriate width and length, and the hollow-fiber membranes in the fabrics run in the warp direction. In the fabrics, the hollow-fiber membranes are spaced from each other in a parallel arrangement by monofilament weft threads, for example.
These arrangements of superimposed hollow-fiber membrane layers are initially produced on appropriate apparatus by stacking layers of the two groups of hollow-fiber membranes over one another in an alternating fashion. In the middle section, the hollow-fiber membranes of the two groups are parallel to each other, and the end regions of the layers are diverted in an alternating manner, separated by group, to the sides in a Y shape. After separately combining the respective end regions, such as with adhesive tape, the finished stack is drawn into the tube-shaped middle section of a housing and the end regions are introduced into the arms of Y-shaped end pieces and embedded therein.
Although the membrane modules according to EP-A-515-034, in comparison to those according to EP-A-514 021, have a higher order and more uniform spacing of the hollow-fiber membranes of the first group from those of the second group, the production of these membrane modules is complicated by the separate production of the hollow-fiber membrane stacks, and the handling of stacks with relatively large cross-section is difficult. Furthermore, drawing the stacks into the housing is problematic, especially in the case of relatively long housings, and the danger exists in this case that the previously realized high order of the hollow-fiber membranes is nullified. Moreover, due to the production technique, there are restrictions with respect to filling ratio. Because of the restrictions with respect to the size of the stack cross-sections and the housing lengths, the module and production concept described for EP-A-515 034 ultimately permits only membrane modules with comparatively small membrane exchange surfaces. Furthermore, the manufacturing complexity of the membrane modules according to EP-A-515 034 increases rapidly if, for example, the system is to be expanded to more than two groups of hollow-fiber membranes or special arrangements of the group of hollow-fiber membranes are needed, and special demands are placed on directing the flow through the hollow-fiber membranes of the individual groups.
This also applies to the membrane modules according to EP-A-515 033, which differ from those of EP-A-515 034 in that the membrane stacks consist of at least one pair of superimposed fabric strips made from hollow-fiber membranes, where the fabric strips are folded with each other parallel to the hollow-fiber membranes in a zigzag pattern and superimposed to form a stack.